Endothermic chemical conversions of hydrocarbons and other reactants have been extensively practiced in the prior art. The large investment capital requirements and operating costs associated with such endothermic processes are often related to poor selectivity and low conversion typical of those processes, or to the heat requirements of the processes. Such processes are usually ultimately limited in conversion by an unfavorable position of equilibrium. It is desirable therefore to favorably change the equilibrium by continuous removal of one of the reaction products, and to provide heat required for the process by the conduct of an exothermic reaction simultaneously with the endothermic reaction within the same reactor train. The present invention provides a selective process with improved conversion and consequent lower installation and operating costs while also providing major benefits associated with in situ heat generation and equilibrium shift to achieve higher single-pass conversion.
Some of the commercially used endothermic processes of the prior art use adiabatic reactors, with the resulting disadvantage that the requisite reactor size to approach equilibrium conversion becomes quite large due, to the rapid deceleration of the reaction as the reaction mixture progresses through the bed of catalyst and the temperature drops with increasing conversion of the reaction mixture. An isothermal reactor would require less volume to achieve an equivalent conversion. The present invention makes it possible to conduct endothermic reactions while avoiding the disadvantages of adiabatic reactors.
It has been proposed to conduct endothermic dehydrogenation processes in the presence of oxygen in order to react the hydrogen produced in the dehydrogenation with oxygen to provide a continuous shift of the equilibrium to higher levels and to provide heat for endothermic reaction. However, a practical problem associated with endothermic processes which are conducted in the presence of oxygen in order to oxidize: product hydrogen in situ is that introduction of oxygen which has not been previously diluted with inert gas into the reactor may cause the production of oxygen-fuel mixtures within the bounds of the combustion envelope at temperatures above the autoignition temperature. This may lead to unselective combustion of desired hydrocarbon products or to explosion in an extreme case.
To mix oxygen with hydrocarbons safely in a packed bed requires a large expense. To avoid bulk mixing of oxygen and hydrocarbons, a hydrogen combustion catalyst in the form of a porous ceramic monolith honeycomb, in which oxygen is diffused through the pores of the monolith and the hydrocarbon stream is passed through the tubular channels of the honeycomb over a selective hydrogen combustion catalyst, might be used. In such a system, oxygen and hydrogen would not mix in the bulk except at the interface over the catalyst where selective combustion of hydrogen occurs. This design ameliorates the safety concerns but introduces a heat transfer problem. To effectively transfer heat from the hydrogen combustion zone into the catalyst beds in which the endothermic dehydrogenation reaction proceeds, high wall temperatures are required which may exceed the temperature at which rapid thermal coking of the feed occurs. This is a problem common to all the commercial dehydrogenation processes which use a packed bed reactor for dehydrogenation and not just those which rely on in situ hydrogen combustion to provide heat. But even if hydrogen is oxidized in situ to make the overall process thermoneutral, unless the exothermic combustion process can be conducted in the same zone as the endothermic dehydrogenation, heat transfer limitations may limit efficiency. The present invention provides a way to avoid heat transfer problems while achieving advantages of the use of a porous honeycomb catalyst whether such honeycomb catalyst is used or not.
Imai in U.S. Pat. Nos. 4,435,607 and 4,788,371 discloses dehydrogenation of alkanes in processes which include selective hydrogen combustion by oxygen, but have the disadvantage of adding oxygen to a zone in which there is either a high concentration of combustible organic compounds necessitating a high concentration of diluent such as steam to prevent combustion of organic compounds.
Clark et al U.S. Pat. No. 5,124,500 discloses a process for the removal of hydrogen from a mixture of hydrogen and organic compounds by selective reaction of the hydrogen with a molecular sieve containing a reducible metal cation, and discloses one type of material that could be used in the reactor design embodiment of the present invention. No specific reactor concept is disclosed by Clark, et al, nor any concept of the importance or difficulty of efficiently using the exotherm associated with hydrogen oxidation to provide heat for the dehydrogenation.